Abstract
When Bonamia ostreae (Haplosporidia) was first detected in New Zealand flat oysters ( Ostrea chilensis ) in 2015, it threatened one of the world's largest remaining wild flat oyster fisheries in Foveaux Strait and a developing aquaculture sector. Following the subsequent spread of the parasite within New Zealand from the index site (Marlborough Sounds) to Big Glory Bay, Stewart Island, directly adjacent to Foveaux Strait, New Zealand adopted a precautionary strategy to protect the wild fishery by enforcing the depopulation of virtually all farmed O. chilensis . This action aimed to reduce host density and parasite propagule pressure, alongside existing movement controls and targeted surveillance. Here, we describe the biosecurity response since its inception in 2015, identify the key decisions and surveillance design features that most influenced outcomes, highlight constraints, particularly fragmented data systems, and discuss potential mitigation strategies such as resilience breeding and the associated biosecurity considerations.